Hillary Clinton was "extremely careless" in her use of a private email server while she was U.S. secretary of state, but the FBI isn't recommending any charges be brought against her for mishandling classified information.

Speaking to reporters on Tuesday morning, days after the U.S. Federal Bureau of Investigation interviewed the presidential candidate, FBI Director James Comey said his investigation had uncovered numerous instances of the system being improperly used for classified information. He also said it was impossible to rule out the possibility that the system could have been hacked.

The controversy was thought to revolve around a single server, but Comey disclosed that over the years Clinton was secretary of state, she relied on a string of email servers. When new ones were installed, the older ones were removed but data was not always deleted.